Solar cell employing phosphorescent materials

ABSTRACT

A solar cell device having a solid state light absorber region that incorporates a donor-acceptor particle structure. The particle structure includes acceptor particles that generate a flow of electrons in the solid state light absorber region in response to absorbed photons; and donor particles comprising a phosphorescent material, wherein each donor particle is coupled to a group of acceptor particles, and wherein the phosphorescent material absorbs high energy photons and emits lower energy photons that are absorbed by the acceptor particles.

CROSS REFERENCE TO RELATED APPLICATION(S)

This application is a divisional application of and claims prioritybenefit of co-pending U.S. patent application Ser. No. 15/547,186, filedFeb. 11, 2016, which claims priority benefit of U.S. Provisional PatentApplication No. 62/115,667, filed Feb. 13, 2015, the contents of whichare incorporated by reference as if disclosed herein in theirentireties.

FIELD

The subject matter of this invention relates to solar cells employingphosphorescent materials, and more particularly to solar cells having asolid state absorber region that integrates phosphor particles withlight acceptor particles.

BACKGROUND

Converting solar energy to electricity is the one of the cleanestmethods of producing useable energy. Among the various renewable energytechnologies available, solar cells hold significant promise. One majordrawback of this technology is the lack of flexibility, e.g., solarcells only have the ability to generate power during daylight hours andthus cannot provide an uninterrupted power supply without incorporatingadditional components. Another drawback is the relatively low efficiencyof known devices. Hence, a solar cell with improved efficiency that cangenerate power during non-daylight hours and/or that enables efficientstorage of energy generated during daylight hours is desirable.

SUMMARY

The disclosed solution describes a solar cell device that has improvedefficiency in converting light to electrical energy and the ability tooutput power in the dark after a period of excitation in light. Thedevice incorporates a phosphorescent material within controlledproximity of the light absorber used in solar cells. The phosphorescentmaterial is designed and synthesized so as to match its emissionwavelength with the absorption spectrum of the light absorber.

The aforementioned phosphorescent material comprises a donor chromophore(donor particles) that absorbs high energy photons of solar light andemits light of low energy photons over extended rime periods. Thetransfer of energy from the phosphorescent material to the absorber(acceptor particles) results in the generation of additionalelectron-hole pairs in the solar cell as compared to those produced inthe absence of the phosphorescent material, and leads to an improvementin the efficiency of the solar cell. A device comprising the energytransfer system described herein can be adapted for various solar celltechnologies.

A solid state light absorber region is provided that includes adonor-acceptor particle structure having acceptor particles adsorbed ona large surface area of inert nanoparticles that serve as currentcollectors. Upon excitation, a generation, injection, and flow ofelectrons in the solid state light absorber region results in responseto absorbed photons. Donor particles are provided comprising aphosphorescent material that are coupled to groups of acceptor particlessuch that the phosphorescent material absorbs high energy photons andemits lower energy photons that are absorbed by the acceptor particles.

In a first aspect, the invention provides a solar cell device,comprising: a solid state light absorber region that includes adonor-acceptor particle structure having: acceptor particles adsorbed onan inert nanoparticles current collector, which causes a flow ofelectrons in the solid state light absorber region in response toabsorbed photons; and donor particles comprising a phosphorescentmaterial, wherein each donor particle is coupled to a group of acceptorparticles, and wherein the phosphorescent material absorbs high energyphoton and emits lower energy photons that are absorbed by the acceptorparticles.

In a second aspect, the invention provides a dye sensitive solar cell(DSSC) device, comprising: a counter electrode; an electrolyte region; atransparent back contact; and a transparent electrode disposed betweenthe electrolyte region and transparent back contact, wherein thetransparent electrode includes a donor-acceptor particle structurehaving: acceptor particles adsorbed on an inert nanoparticles currentcollector, which upon absorption of photons, results in a flow of freeelectrons in the acceptor particles, injection of electrons into theinert nanoparticles current collector, and transport of injectedelectrons by the inert nanoparticles current collector to thetransparent back contact; and donor particles comprising aphosphorescent material, wherein each donor particle is coupled to agroup of acceptor particles, and wherein the phosphorescent materialabsorbs high energy photons and emits lower energy photons that areabsorbed by the acceptor particles.

In a third aspect, the invention provides a method of forming a dyesensitive solar cell (DSSC) device, comprising: providing a first and asecond transparent back contact; forming a transparent electrode on thefirst transparent back contact wherein the transparent electrodeincludes a donor-acceptor particle structure having: acceptor particlesadsorbed on an inert nanoparticles current collector; and donorparticles comprising a phosphorescent material, wherein each donorparticle is coupled to a group of acceptor particles, and wherein thephosphorescent material absorbs high energy photons and emits lowerenergy photons that are absorbed by the acceptor particles; forming acounter electrode on the second transparent back contact; and forming anelectrolyte region between the counter electrode and transparentelectrode; wherein the transparent electrode forms a light absorptionregion, which upon absorption of photons results in a flow of freeelectrons in the acceptor particles, injection of electrons into theinert nanoparticles current collector and transport of injectedelectrons by the inert nanoparticles current collector to the firsttransparent back contact.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of this invention will be more readilyunderstood from the following detailed description of the variousaspects of the invention taken in conjunction with the accompanyingdrawings in which:

FIG. 1 depicts a donor-acceptor particle structure according toembodiments.

FIG. 2 depicts a dye sensitive solar cell according to embodiments.

FIG. 3 depicts a graph showing spectral matching of donor and acceptorparticles according to embodiments.

The drawings are not necessarily to scale. The drawings are merelyschematic representations, not intended to portray specific parametersof the invention. The drawings are intended to depict only typicalembodiments of the invention, and therefore should not be considered aslimiting the scope of the invention. In the drawings, like numberingrepresents like elements.

DETAILED DESCRIPTION

A solar cell device and associated method for forming the device aredisclosed having a solid state light absorber region or chromophore thatincludes coated phosphor “donor” particles placed in close proximity toassociated “acceptor” particles. While both types of particles act aschromophores (light absorbers), the donor particles include aphosphorescent material that absorbs high energy photons of solar lightand emits light of low energy photons over extended time periods. Thephosphorescent material is selected so as to enable spectral overlap ofits emission wavelength with the absorption spectrum of the acceptorparticles. The transfer of energy from the phosphorescent material tothe acceptor particles results in the generation of additionalelectron-hole pairs in the absorber chromophore as compared to thoseproduced in the absence of the phosphorescent material, and leads to theimprovement in the efficiency of the solar cell. In addition, becausethe phosphorescent material continues to emit low energy photons evenafter the removal of an excitation light source (e.g., the sun in thepresent case), the resulting solar cell device has the ability to outputpower in the dark after a period of excitation in light.

In an illustrative embodiment, the donor-acceptor particle structure isimplemented in a solid state form with inorganic phosphor particleshaving a very high phosphorescence efficiency in which the distancebetween each donor particle and associated acceptor particles iscarefully controlled to optimize energy transfer between the two. Theuse of inorganic phosphorescent materials allows emission of radiationfor many hours in darkened conditions, which can be used to power asolar cell device in the absence of light. Experimental results in a dyesensitive solar cell application show approximately a 60% improvement inthe solar cell device's efficiency under illumination using simulatedsunlight and 300 times improvement in the dark. Because the energytransfer is achieved in a solid state, the present approach is notrestricted to organic phosphors embedded in liquid electrolytes, such asphotoelectrochemical (PEC) solar cells, but can instead be adapted for awide variety of known solar cells.

Referring now to the drawings, FIG. 1 depicts a donor-acceptor particlestructure 10 that includes a phosphor (donor) particle 16 having acoating or spacer 14 coupled to a group of acceptor particles 12. Theacceptor particles 12 are adsorbed on inert nanoparticles that serve ascurrent collectors, the structure generally being referred to as an“inert nanoparticles current collector” 15. When incorporated into asolar cell device, the phosphorescent material that makes up thephosphor particle 16 is excited by high energy photons of sunlight,which is transferred to the acceptor particles 12 to both enhanceefficiency during lighted conditions and allow for extended periods ofuse (e.g., several hours) in darkened conditions.

FIG. 2 depicts an illustrative embodiment of a dye sensitized solar cell(DSSC) device 20 that employs the abovementioned donor-acceptor particlestructure 10. Device 20 generally includes a first transparent backcontact 22, a top electrode 24 (i.e., anode) that incorporates thedonor-acceptor particle structure 10 and inert nanoparticles currentcollector 15 to form a solid state light absorbing region, anelectrolyte 26, a counter electrode 28 (cathode) and a secondtransparent back contact 30. The illustrative solar cell device 20 isgenerally implemented using a standard dye sensitized solar cell (DSSC)architecture, with the additional phosphor material incorporated withthe acceptor particles in the top electrode 24.

In the illustrative DSSC embodiment 20, phosphor particles 16 may forexample comprise SrAl₂O₄ coated with a thin layer of polycrystallineTiO₂ film that forms the spacer 14. Acceptor particles 12 may forexample be comprised of an absorber such as dye or quantum dot materialadsorbed on a TiO₂ nanoparticle current collector. The TiO₂ film (i.e.,spacer 14) prevents the direct contact of the phosphor material 16 withthe electrolyte 26, thus preventing the quenching of phosphorescentsignals by the iodide/triiodide or other redox couples. The spacer 14also serves as a barrier against direct charge transfer between thephosphor particle 16 and the acceptor particles 12, and serves as anelectrical conduit for injected electrons from the excited dye to theback contact 30 of the solar cell. TiO₂ is transparent to the visiblelight (e.g., λ=520 nm) emitted by the phosphor particles 16. In oneillustrative embodiment, the thickness of the TiO₂ spacer may be in therange of 8-10 nm.

DSSC device 20 generally includes a solid dye structure formed withinthe top electrode 24 for catching photons of incoming light, whichconvert the energy into excited electrons. The excited electrons arethen injected into a conduction band of the TiO₂ nanoparticles in theelectrode 24, and conducted away (upward In FIG. 2 ) to the firsttransparent back contact 22 by the layer's TiO₂ nanoparticles currentcollector 15 (FIG. 1 ). Electrolyte 26 closes the circuit 32 allowingthe electrons to return to the dye within electrode layer 24. Themovement of the electrons through the electrical circuit 32 can be usedto run electrical devices and thus produce usable work. By incorporatingthe donor-acceptor particle structure 10 into the top electrode 24, thenumber of electron hole pairs is increased, thus improving efficiency.Furthermore, because the phosphorescent material continues to emitenergy for many hours after excitation, the device will continue tooutput electricity in darkened conditions.

Although described in a DSSC embodiment, the described donor-acceptorparticle structure 10 may likewise be incorporated into other types ofdevices, such as quantum dot solar cells, polymer solar cells, and thinfilm solar cells.

In the case of a DSSC device, the following illustrative fabricationprocess may be utilized, with reference to FIGS. 1 and 2 . First, alight emitting phosphor 16 is selected, such as a green(Sr_(0.95)Eu_(0.02)Dy_(0.03)Al₂O₄) or blue(Sr_(2.95)Eu_(0.03)Dy_(0.07)Al₄O₁₁) emitting phosphor having particlediameters of approximately 230 mesh and 300 mesh respectively. Next, thephosphor particles 16 are coated with a film (i.e., spacer) 14. This mayfor example be done by repeatedly spraying a mixture of 0.1 M titanium(IV) isopropoxide and 1.2 M acetylacetonate in ethanol on the phosphorparticles 16 dispersed on a silicon wafer maintained at 400° C. Afterthe deposition, the particles may be annealed at 500° C. for 30 minutesto remove trace organics. The thickness of the coating may be in rangeof 8-10 nm, which may he accomplished with 40 deposition cycles.

TiO₂ nanoparticles may be prepared as a transparent TiO₂ paste withhydrothermal synthesis used titanium (IV) isopropoxide as a precursor.For example, the procedure may utilize the drop wise addition of 3.7 mLof the precursor to a breaker containing a mixture of 1 mL of2-propanol, 8 mL of acetic acid, and 25 mL distilled water kept over anice bath.

The mixture can then be heated at 80° C. for 25 minutes. The entirecontents can then be transferred to an autoclave and heated to atemperature of 250° C. for approximately 13 hours. The resultingparticles are repeatedly washed, with distilled water, centrifuged, andfinally suspended in ethanol until further use. Particles obtained bythis procedure comprise pure anatase phase of TiO₂ with an averageparticles size of ˜20 nm. Scattering TiO₂ nanoparticles may be prepared,by dispersing 10 grams of P25 (Degussa) powder in 30 mL of ethanol. Themixture may then be sonicated for 30 minutes.

The top electrode 24 may be fabricated as follows. First, afluorine-doped tin oxide coated glass (FTO substrate—i.e., firsttransparent back contact 22) is coated with a transparent blocking layerof TiO₂, such as the provided by SOLARONIX®, and annealed in a furnaceat 500° C. for 30 minutes, which is then followed with a deposition ofthick transparent TiO₂ nanoparticles. The film is then dried and coatedwith 150 μm of paste containing scattering TiO₂ nanoparticles and TiO₂coated SrAl₂O₄. The volumetric ratio of TiO₂ to phosphor particles mayfor example be in the range of approximately 10:1. The film is thenannealed at 500° C. for 60 minutes to achieve a total film thickness ofapproximately 100 microns. Before sensitization with ruthenium dye(e.g., N719, SIGMA ALDRICH®), the substrate is immersed in 0.02M TiCl₄solution for 10 minutes, and annealed again at 450° C. for 60 minutes.The resulting substrate is then immersed in a 0.5 mM N719 dye dissolvedin 1:1 (v/v) ratio of acetonitrile and tert-butyl alcohol at 4° C. toform the top electrode 24.

A platinum counter electrode 28 may be prepared by applying a thincoating of chloroplatinic acid on a second FTO substrate (i.e., secondtransparent back contact 30), followed by annealing at 500° C. for 1hour.

The electrodes 24, 28 may be assembled using a 200 μm-thick hot-meltfilm (Surlyn 1702, SOLARONIX®) as spacer between the transparentelectrode 24 and counter electrode 28. An electrolyte 26, e.g.,consisting of 0.6 M PMII, 0.05 M I₂, 0.05M tertbutyl pyridine (0.04 M)and 0.025M guanidinium thiocyanate in 4:1 (v/v) ratio of acetonitrileand valeronitrile, may be injected through a small, predrilled hole inthe counter electrode 28.

As noted herein, the emission spectra of the donor particles should beselected to overlap the absorption spectra of the acceptor particle tomaximize efficiency of the device. FIG. 3 , for example, shows theabsorption and emission spectra of N719 dye dissolved in tert-butylalcohol along with the emission spectra of SrAl₂O₄ (dispersed on acopper foil) recorded at room temperature at an excitation wavelength of373 nm. The absorption spectrum of N719 dye shows an absorption peak at520 nm and has a good spectral overlap with the emission range ofSrAl₂O₄, which is necessary for efficient excitation energy transfer.SrAl₂O₄ has a broad excitation spectrum from 460-250 nm, and thus can beexcited by solar radiation. The phosphor exhibits a broad bandphotoluminescence (PL) with maximum intensity occurring at 520 nm.

The foregoing description of various aspects of the invention has beenpresented for purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formdisclosed, and obviously, many modifications and variations arepossible. Such modifications and variations that may be apparent to anindividual in the art are included within the scope of the invention asdefined by the accompanying claims.

What is claimed is:
 1. A method of forming a dye sensitive solar cell(DSSC) device, comprising: providing a transparent front contact;providing a transparent back contact; forming donor particles that areeach coated with a spacer layer, the forming comprising: providingphosphorescent particles dispersed on a silicon wafer maintained at 400°C.; spraying a mixture of 0.1 M titanium (IV) isopropoxide and 1.2 Macetylacetonate in ethanol on the phosphorescent particles; annealingthe phosphorescent particles at 500° C. for 30 minutes; and repeatingthe spraying and annealing steps for a total of 40 cycles such that eachphosphorescent particle is coated with the spacer layer having athickness in the range of 8 nm to 10 nm; forming a transparent electrodeon the transparent front contact, the forming comprising: providing aninert nanoparticles current collector; mixing the donor particles withthe inert nanoparticles current collector to form a transparent paste;applying the transparent paste to a surface of the transparent frontcontact to form a transparent film; annealing the transparent film at500° C. for 60 minutes such that the transparent film has a thickness ofapproximately 100 microns; immersing the transparent front contact in0.02M TiCl₄ solution for 10 minutes; annealing the transparent frontcontact at 450° C. for 60 minutes; and immersing the transparent frontcontact in an acceptor particles dye such that the acceptor particlesare adsorbed on the inert nanoparticles current collector; wherein thetransparent electrode includes a donor-acceptor particle structurewherein each donor particle is coupled to a group of acceptor particles,and wherein the phosphorescent material absorbs high energy photons andemits lower energy photons that are absorbed by the acceptor particles;forming a counter electrode on the transparent back contact; and formingan electrolyte region between the counter electrode and transparentelectrode; wherein the transparent electrode forms a light absorptionregion, which upon absorption of photons results in a flow of freeelectrons in the acceptor particles, injection of electrons into theinert nanoparticles current collector, and transport of injectedelectrons by the inert nanoparticles current collector to thetransparent front contact.
 2. The method of claim 1, wherein theacceptor particles dye comprises a ruthenium dye and the inertnanoparticles current collector comprises pure anatase phase of TiO₂nanoparticles having an average particle size of 20 nm.
 3. The method ofclaim 1, wherein the spacer layer comprises TiO₂.
 4. The method of claim1, wherein a volumetric ratio of acceptor particles to donor particlesis approximately 10:1.
 5. The method of claim 1, wherein thephosphorescent particles comprise green light emitting phosphorescentparticles having a particle diameter of approximately 230 mesh.
 6. Themethod of claim 1, wherein the phosphorescent particles comprise bluelight emitting phosphorescent particles having a particle diameter ofapproximately 300 mesh.
 7. The method of claim 1, wherein forming theelectrolyte region comprises injecting an electrolyte through thecounter electrode.